As power source for portable apparatuses and the like, lithium secondary batteries are mainly used. As functions of portable apparatuses and the like become more and more diversified, power consumption of each apparatus is on the increase. Further, such an apparatus is required to have quick-charge characteristics. Therefore, a lithium secondary battery is required to have an increased battery capacity and at the same time an improved charge-discharge cycle characteristics, and furthermore to have an enhanced quick-charging property.
In such a lithium secondary battery, generally, lithium-containing metal oxide such as lithium cobaltate is used as cathode active material and carbonaceous material such as graphite is used as anode active material.
Carbonaceous materials can be roughly categorized into graphite material with a high crystallinity degree and amorphous carbon material with a low crystallinity degree. Both types, which allow lithium insertion/elimination reaction, can be used as anode active material.
Amorphous carbon material is known to have a good charge characteristics while it has a disadvantage of significant cycle deterioration. On the other hand, highly crystalline graphite material has a stable cycle characteristics while its charge characteristics is inferior to that of amorphous carbon material. Currently, graphite materials having stable cycle characteristics are widely used as negative electrode material.
Various techniques have been proposed to achieve a high battery capacity, good cycle characteristics and charge characteristics in graphite material serving as anode active material.
For example, Patent Document 1 discloses a method for obtaining a graphite material, where fine particles with a low volatile matter content which have been obtained by thermally treating a coal-based pitch containing 1.0% or less of free carbon and pulverizing followed by classification, are mixed with fine particles with a high volatile matter content which have been obtained by thermally treating a coal-based pitch to prepare a thermal decomposition product having a softening point of 320 to 380° C., pulverizing the thermal decomposition product, classifying it and non-solubilizing it, and then the mixture is subjected to calcination to be graphitized to thereby obtain a graphite material (Comparative Example 1 described later).
Patent Document 2 describes about massive-form graphite particles consisting of scaly natural graphite particles and proposes controlling D-50% in the particle size distribution to be 10 to 25 μm, specific surface area to be 2.5 to 5 m2/g and particulate bulk density to be in a predetermined range (Comparative Example 2 described later.)
Patent Document 3 discloses about coating the surface of graphite powder with a carbon precursor and calcinating it in an inert gas atmosphere at a temperature range of 700 to 2800° C. (Comparative Example 3 described later).
Further, Patent Document 4 discloses using composite graphite particles as an anode active material, which is obtained by granulating a scaly graphite having d(002) of 0.3356 nm, R value of around 0.07 and Lc of about 50 nm by use of an external mechanical force to thereby prepare spheroidized graphite particles and coating the particles with a carbide obtained by heating a resin such as phenol resin. The document teaches that the composite graphite particles are obtained by carbonizing in a nitrogen atmosphere preliminarily at 1000° C. and then at 3000° C. (Comparative Example 4 described later).
These conventional graphite materials all exhibit a high battery capacity. The cycle characteristics of these materials, except for the case of Patent Document 1, however, are insufficient. Moreover, charge characteristics in the materials are all low.                [Patent Document 1]Japanese Patent Application Laid-Open No. 2003-142092        [Patent Document 2]Japanese Patent Application Laid-Open No.2002-237303        [Patent Document 3]Japanese Patent No. 3193342 (European Patent No. 917228)        [Patent Document 4]Japanese Patent Application Laid-Open No.2004-210634 (WO2004/056703 pamphlet)        